In some databases, a lower level class inherits properties from an upper level class, such as in Object-Oriented Databases (OODB) and Object-Relational Database (ORDB). The function wherein a property of the upper level class is inherited to the lower level class is called “inheritance”. For example, inheritance is disclosed in “Object-Oriented Concepts, Databases, and Applications, Edited by Won Kim, 1989, ACM Press”.
In references disclosing the Object-Oriented Database, classification in hierarchical structure is called “class”. On the other hand, in the Object-Relational Database, a table of which property is inheritable corresponds to a class. In a plurality of tables having hierarchical structure, a property is inherited from an upper level table to a lower level table. Briefly, header information of column comprising the upper level table is inherited to the lower level table. In this case, both databases are called “hierarchical database”. A value of the same property belonging to each hierarchical class is called “instance” or “contents data”, and a set of the data is called “population”.
In Internet, as international standard to package an electronic catalogue system providing product information, ISO13584 (Parts Library) is well known. This is one of the hierarchical databases. In ISO13584, an electronic catalogue is comprised of schema and contents. By providing data structure unifying the schema and contents, common use and reuse of product information can be realized.
In the schema defined in ISO13584, product classification is hierarchically represented by single tree structure of “product class”. Each “product class” has “property item”, and “property item” of some “product class” is inherited to “product class” of lower level. Furthermore, in order to uniquely specify “product class” and “property item”, a unique ID called “BSU code” is respectively assigned. On the other hand, the contents are represented as a table in which a property value peculiar to each product is embedded in property item defined in the schema.
While ISO13584 provides a framework as the electronic catalogue, international standardization of actual schema is progressed. In IEC61360, an upper level hierarchical part of schema in electric/electronic area, i.e., a general part of “product class” and “property item”, are progressed. By using the standardization, product catalogue creator of each company can determine original detail “product class” and “property item” as lower level expansion from IEC61360, and create its own contents. By utilizing the contents, a user of the electronic catalogue can trace a class hierarchy of “product class”, restrict to a product necessary for the user by referring to the property value, and retrieve the desired product. Recently, in response to this standardization, several systems based on ISO13584 are developed.
In Relational database or ORDB, a join operation (JOIN) as one operation between tables is prepared. In the hierarchical database, this operation can be executed. In general, the join operation (JOIN) is simple, but time consuming because of occurrence of loop of “(the number of rows)×(the number of rows)”. Accordingly, in order to provide the join operation (JOIN) to a user in a Web type database system, the number of operations can not be controlled. In other words, the join operation (JOIN) having a large number of operations and a large system load often occurs.
As mentioned-above, in the hierarchical database having a hierarchical structure of a plurality of classes, when joining a contents data table registered in a class of the hierarchical structure with a table stored in the same or another table, the number of operations can not be controlled. Accordingly, a join operation (JOIN) of large system load occurs.